1. Field of the Invention
The present invention relates to a novel end-capping agent for an addition curable heterocyclic compound for a heat-resistant resin. More particularly, it relates to an end-capping agent having an addition-reactive alicyclic chemical structure. Furthermore, the present invention relates to an easily-moldable polyquinoxaline resin having excellent heat resistance and adhesive strength to reinforcing fibers. More particularly, it relates to a thermosetting quinoxaline resin oligomer which is addition-curable by an alicyclic endo compound present at both ends.
2. Description of the Related Art
With recent rapid progress in space and aircraft industries and electronic instrument, the demand for heat-resistant polymers is rapidly increasing. In this respect, the development of fiber-reinforced composite materials represented by carbon fiber reinforced composites is particularly noticeable.
At the present, a polyimide resin is most widely used as the highly heat-resistant matrix resin for a fiber-reinforced composite. However, since the polyimide resin has an insufficient thermal oxidative stability at high temperatures, or water resistance, the development of a heat-resistant resin free from these problems, which can be used instead of the polyimide resin, is eagerly desired.
A polyquinoxaline resin developed by Hergenrother et al. (J. Polym. Sci. A-1 15, 1453 (1967)) has a superior thermal oxidative stability and water resistance to those of the polyimide resin, and it is therefore expected that this resin will be used as a matrix resin for a high-performance fiber-reinforced composite. However, since a high molecular weight polyquinoxaline resin has a high melt viscosity, it is difficult to impregnate this resin into a reinforcing fiber, and therefore, a high temperature and a high pressure are necessary for the molding. As means for improving the moldability, a method has been examined in which an oligomer having an addition-reactive end group, as is well-known in the field of the polyimide resin.
According to this method, the melt viscosity of the resin is reduced and the flowability of the resin during the cure cycle is improved, and therefore, the molding is facilitated.
For example, there can be mentioned an addition-curable quinoxaline resin oligomer which has an acetylene end group represented by the following formula XII (U.S. Pat. No. 3,975,444), and an addition-curable quinoxaline resin oligomer which has a nitrile end group represented by the following formula XIII (SAMPE, vol. 8, page 114 (1976)). However, the reaction temperature of these oligomers is as high as 350.degree. C. to 400.degree. C., molding is difficult. ##STR4##
A quinoxaline oligomer represented by the following formula XIV, in which an alicyclic group is introduced (U.S. Pat. No. 3,784,307 and U.S. Pat. No. 3,748,310) has been studied, as for a low temperature addition curable quinoxaline resin. ##STR5##
Although these resins have an improved moldability, there are some problems as composite materials in that the mechanical properties such as flexural strength and interlaminar shear strength thereof are poor.